1. Field of the Invention
This invention relates to a method of driving a brushless dc motor. This invention also relates to an apparatus for driving a brushless dc motor.
2. Description of the Prior Art
General brushless dc motors include a stator having windings and a rotor having permanent magnets. The stator windings are supplied with drive currents. To drive the rotor satisfactorily, it is necessary to sequentially change the drive currents in response to the angular relation (the positional relation) between the stator windings and the magnetic poles of the rotor.
Some of prior art apparatuses for driving a brushless dc motor include position sensors detecting the angular relation (the positional relation) between stator windings and magnetic poles of a rotor. Drive currents fed to the stator windings are sequentially changed in response to the output signals from the position sensors.
Japanese published examined patent application 61-3193 discloses a prior art advanced apparatus for driving a brushless dc motor which dispenses with position sensors. This prior art advanced apparatus uses the following fact. In a brushless dc motor, as a rotor rotates, voltages are induced in stator windings in dependence on the positional relation between the stator windings and magnetic poles of the rotor. The induced voltages are opposite to drive voltages related to drive currents fed to the stator windings. The induced voltages are generally referred to as "back" voltages. In the prior art advanced apparatus of Japanese patent application 61-3193, the induced voltages are detected as an indication of the positional relation between the stator windings and the magnetic poles of the rotor, and the detected voltages are used for the control of the change of the drive currents. The prior art advanced apparatus of Japanese patent application 61-3193 tends to be complicated in structure.